The present invention relates to an electrophotographic copier, facsimile apparatus or similar image recorder of a type for developing an electrostatic latent image which is formed on a photoconductive element, or image carrier, by using a developer that is implemented by a toner. More particularly, the present invention is concerned with a device applicable to such an image recorder for controlling the density of the toner used to form the printed image for a desired image density.
In an image recorder of the type described, the density of developed images is effected by the charge potential deposited on the image carrier, toner density, bias potential for development, etc. Among various image recorders to which the present invention pertains, an electrophotographic copier uses an image carrier in the form of a photoconductive element and, in operation, uniformly charges the surface of the photoconductive element, electrostatically forms a latent image on the charged surface by exposing the surface with an image. This is then developed as the latent image by use of a toner. The device then transfers the resulting toner image to a paper sheet. In this kind of copier, the image density is further effected by the intensity of exposure of the image. It has therefore been customary to maintain the toner density as constant as possible and to allow a person to adjust the charge potential on the image carrier before exposure, the intensity of exposure, the bias potential for development and the like in matching relation to desired image density. A toner density control device for maintaining the toner density constant has been proposed in various forms, as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 57-136667 by way of example. In the device of this Laid-Open Patent Publication, an electrostatic latent image is formed in a predetermined pattern on an image carrier and then developed by a toner which is fed from a developing unit to produce a toner pattern. The disclosed device includes a pattern density sensor responsive to the density of the toner pattern, a toner density sensor responsive to the density of a toner which is stored in the developing unit, and a toner supply device for supplying fresh toner to the developing unit. The device further includes a controller which sets up a toner density basis in association with the pattern density sensed by the pattern density sensor and, when the toner density sensed by the toner density sensor is lower than the toner density basis, actuates the toner supply device to supply a fresh toner to the developing unit.
As stated above, the charge potential on the image carrier and the intensity of exposure of the image and, therefore, the image density is adjusted on the assumption that the toner density is maintained constant at all times, thereby insuring stable image density. However, the adjustable range of image density is limited, i.e., it cannot be varied beyond a certain range which is set up at the production stage. It is therefore impossible for a user or a serviceman to increase or decrease the image density beyond the preset adjustable range. More specifically, a user expecting a variety of features of this type an image recorder may sometimes desire image density higher than the preset upper limit and sometimes image density lower than the preset lower limit for the purpose of saving toner. A machine capable of meeting as many of such demands as possible is useful. However, a desired image density which is either higher or lower than a designed range is not attainable unless various units constituting the image recorder are changed in both of hardware and process conditions. In such a situation, the user has to use another image recorder with which the desired density is available. The prior art control device, therefore, limits the applicable range of image recorders of the type described.